Serían las seis cuando salieron.
It must have been six o’clock when they left.17.7 (p. 77); 48.1.12 (p. 288)
With the conditional perfect, the speaker expresses probability in relation to
something in the past which is previous to another past situation.Estaban muy callados. Seguramente habrían estado discutiendo.
They were very quiet. They had (very) probably been arguing.
El día que llegué estaba llorando. Habría tenido una mala noticia,
quizá(s).
The day I arrived he/she was crying. Perhaps he/she’d had some bad
news.Expressing possibility, probability or uncertainty, etc. 73.12